In (I), Fig. 1, the eight atoms comprising the fused imidazo/thiadiazolethiadiazole residue are co-planar (r.m.s. deviation = 0.009 Å). This system forms dihedral angles of 6.01 (10) and 3.28 (11)° with the attached dichlorobenzene and tolyl rings, respectively. The r.m.s. deviation from the least-squares plane for all 25 non-hydrogen atoms is 0.085 Å with maximum deviations of 0.180 (2) and -0.197 (1) for the O1 and Cl1 atoms, respectively. This is consistent with a small twist about the C1—O1 bond with the C7—O1—C1—C2 torsion angle being -168.5 (2)°. The S and O atoms are syn and are separated by 2.823 (3) Å. A small twist was also observed in the structure of the closely related compound 2-isobutyl-6-phenylimidazo[2,1-b][1,3,4]thiadiazole (Fun et al., 2011).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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